Linear alkylbenzene sulfonate (LAS) is one of the most widely used surfactants in commerce. It finds special application for light and heavy duty liquid and powdered detergents. A potential disadvantage of LAS, however, is that under hard water conditions, i.e., calcium levels greater than about 150 parts per million, it can interact with cationic water hardness ions, such as calcium, thereby becoming inactivated through precipitation. While this is a problem common to anionic surfactants, LAS is especially sensitive to water hardness ions.
Although not wishing to be bound by any theory, the literature indicates that the aforementioned interaction can best be understood by considering the micellar structure of anionic surfactants, e.g., LAS. Repulsive forces between negative charges in the sulfonate group lead to a higher critical micelle concentration (CMC) than, for example, with a nonionic surfactant. CMC is the surfactant concentration at which micellar formation begins. Stated otherwise, the negative charge of LAS retards micellar formation and shifts the equilibrium towards the monomer. A relatively high monomer concentration in solution results thereby; this is significant because precipitation between calcium ion and LAS occurs only with the monomer.
It has been found that liquid laundry detergent compositions containing secondary alkyl sulfate as an anionic surfactant component can be used in liquid laundry detergent compositions in place of either LAS or primary alkyl sulfates with the advantages being increased tolerance for water hardness ions, good detergency properties, higher active raw material, i.e., less water, and better color of anionic raw material.